Locking up some of Britain's most notorious killers for life without any prospect of release is a breach of their human rights, European judges have ruled.
Murderer Jeremy Bamber is among those facing a review of their whole-life tariffs after winning an appeal that the sentences were "inhuman and degrading".
The judges found that for a life sentence to remain compatible with the European Convention on Human Rights there had to be both a possibility of release and a possibility of review.
It means the Government must amend the law to ensure it complies with human rights legislation, and opens the door to demands for early release from prisoners who were told they could never walk free.
The panel of 17 judges in the Grand Chamber of the European Court of Human Rights said their decision did not mean Bamber and two other men - Douglas Vinter and Peter Moore - can look forward to being freed in the near future.
They said: "In finding a violation in this case … the court did not intend to give the applicants any prospect of imminent release."
However, the decision has been slammed by many in Westminster - including Prime Minister David Cameron, who said he was "very, very disappointed".
A spokesman said: "He is a strong supporter of whole-life tariffs."
Justice Secretary Chris Grayling said people would find the ruling "intensely frustrating".
"I think this will simply reinforce the desire in this country for wholesale reform of our human rights laws," he told Sky News.
"I suspect that the vast majority of the public think the court is just getting this wrong - at the very least it should be a matter for the British Parliament."
However, Eric Allison, a journalist who spent 15 years serving time in British prisons, said it was a "victory for the possibility" of rehabilitating even the worst of criminals.
"I've seen people who've done some awful things and I've seen them change their character completely in prison," he said.
The appeal was brought by Vinter, who stabbed his wife in February 2008, and means the cases of Bamber - who killed his parents, sister and two young children in August 1985 - and Moore, who killed four gay men in 1995, will also be considered.
Bamber, 51, has been behind bars for more than 25 years for shooting his wealthy adopted parents June and Neville, his sister Sheila Caffell and her twin sons Daniel and Nicholas at their farmhouse in Tolleshunt D'Arcy, Essex.
A statement from Bamber described winning the appeal as "hollow" as he is in prison "for a crime I did not commit".
He added: "My whole life order has now been given a system of reviews, but there is no provision for someone who is wrongly convicted to prove that they are worthy of release, such hope is in reality, no hope at all."
In their ruling the judges said it was up to national authorities to decide when a review should take place, but existing laws point to a review happening no later than 25 years after a life sentence is handed down.
Current UK law says whole-life tariff prisoners will almost certainly never be released because their offences are deemed to be so serious.
Only the Justice Secretary can free them on compassionate grounds - if, for example, they are terminally ill.
The judges said the decision on whether to release prisoners jailed for their entire lives would depend on whether there were legitimate grounds for their continued detention.
Until 2003, whole-life tariff prisoners had a right to a review but this was removed in a change to the law.
Vinter's lawyer Simon Creighton, of Bhatt Murphy Solicitors, said the ruling could not be used as a "get out of jail free" excuse.
He said: "They have not said that anyone must be released, what they have said is that it must be reviewed.
"It's now for the Government to respond."
The ruling comes shortly after Home Secretary Theresa May voiced her frustrations with the European courts in the House of Commons in the wake of the lengthy and costly fight to boot radical cleric Abu Qatada out of the country.
She said she wanted the Human Rights Act itself to be scrapped.